Radio communication system

ABSTRACT

A radio communication system has means for improving power control of a communication channel for the transmission of data after an interruption in the transmission. This is done by adjusting the transmission power immediately after the interruption by an offset from the power used before the interruption. The offset may be fixed or may be determined from the transmission power in the period before the interruption. This technique reduces, on average, the time taken for power control to be re-established, thereby addressing the problem that data transmissions immediately after the interruption are likely to be corrupted if the power level is too low, or to generate extra interference if the power level is too high.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of prior application Ser. No. 09/478,475 filedJan. 6, 2000 now U.S. Pat. No. 6,754,505.

The present invention relates to a radio communication system andfurther relates to primary and secondary stations for use in such asystem and to a method of operating such a system. While the presentspecification describes a system with particular reference to theemerging Universal Mobile Telecommunication System (UMTS), it is to beunderstood that such techniques are equally applicable to use in othermobile radio systems.

There are two basic types of communication required between a BaseStation (BS) and a Mobile Station (MS) in a radio communication system.The first is user traffic, for example speech or packet data. The secondis control information, required to set and monitor various parametersof the transmission channel to enable the BS and MS to exchange therequired user traffic.

In many communication systems one of the functions of the controlinformation is to enable power control. Power control of signalstransmitted to the BS from a MS is required so that the BS receivessignals from different MS at approximately the same power level, whileminimising the transmission power required by each MS. Power control ofsignals transmitted by the BS to a MS is required so that the MSreceives signals from the BS with a low error rate while minimisingtransmission power, to reduce interference with other cells and radiosystems. In a two-way radio communication system power control isnormally operated in a closed loop manner, whereby the MS determines therequired changes in the power of transmissions from the BS and signalsthese changes to the BS, and vice versa.

An example of a combined time and frequency division multiple accesssystem employing power control is the Global System for Mobilecommunication (GSM), where the transmission power of both BS and MStransmitters is controlled in steps of 2 dB. Similarly, implementationof power control in a system employing spread spectrum Code DivisionMultiple Access (CDMA) techniques is disclosed in U.S. Pat. No.5,056,109.

A problem with these known techniques is that at the start of atransmission, or after the transmission is interrupted, the powercontrol loops may take some time to converge satisfactorily. Until suchconvergence is achieved data transmissions are likely to be received ina corrupted state if their power level is too low, or to generate extrainterference if their power level is too high.

An object of the present invention is to address the above problem.

According to a first aspect of the present invention there is provided aradio communication system comprising a primary station and a pluralityof secondary stations, the system having a communication channel betweenthe primary station and a secondary station, the channel comprising anuplink and a downlink control channel for transmission of controlinformation, including power control commands, and a data channel forthe transmission of data, wherein power control means are provided foradjusting the power of the control and data channels in response to thepower control commands and means are provided for setting the initialtransmission power after a pause in transmission to that before thepause adjusted by an offset.

According to a second aspect of the present invention there is provideda primary station for use in a radio communication system having acommunication channel between the primary station and a secondarystation, the channel comprising an uplink and a downlink control channelfor transmission of control information, including power controlcommands, and a data channel for the transmission of data, wherein powercontrol means are provided for adjusting the power of the control anddata channels in response to the power control commands and means areprovided for setting the initial transmission power after a pause intransmission to that before the pause adjusted by an offset.

According to a third aspect of the present invention there is provided asecondary station for use in a radio communication system having acommunication channel between the secondary station and a primarystation, the channel comprising an uplink and a downlink control channelfor transmission of control information, including power controlcommands, and a data channel for the transmission of data, wherein powercontrol means are provided for adjusting the power of the control anddata channels in response to the power control commands and means areprovided for setting the initial transmission power after a pause intransmission to that before the pause adjusted by an offset.

According to a fourth aspect of the present invention there is provideda method of operating a radio communication system comprising a primarystation and a plurality of secondary stations, the system having acommunication channel between the primary station and a secondarystation, the channel comprising an uplink and a downlink control channelfor transmission of control information, including power controlcommands, and a data channel for the transmission of data, and at leastone of the primary and secondary stations having power control means foradjusting the power of the control and data channels in response to thepower control commands, the method comprising setting the initialtransmission power after a pause in transmission to that before thepause adjusted by an offset.

The offset may be predetermined. Alternatively it may be determined fromthe difference between the last transmission power and a weightedaverage of the transmission power over a period (possibly predetermined)before the pause in transmission, or may be determined from a weightedsum of the power control commands applied before the pause intransmission. In such cases the offset should be quantised to anavailable power control step size before it is applied.

The use of more than one power control step size is known, for examplefrom JP-A-10224294. However its use in this citation is limited tosituations where power control is already established but propagationconditions are fluctuating rapidly. This citation does not address theproblem of obtaining rapid convergence of power control at the start of,or after an interruption in, a transmission.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, wherein:

FIG. 1 is a block schematic diagram of a radio communication system;

FIG. 2 illustrates a conventional scheme for establishing acommunication link;

FIG. 3 illustrates a scheme for establishing a communication link havinga delayed start to data transmission;

FIG. 4 is a flow chart illustrating a method for performing powercontrol operations having a variable step size;

FIG. 5 is a graph of received signal power (P) in dB against time (T) inms for different power control algorithms, the solid line indicatingresults with no power control, the chain dashed line indicating resultswith power control having a single step size, and the dashed lineindicating results with power control having two step sizes; and

FIG. 6 is a graph of received signal power (P) in dB against time (T) inms for different power control algorithms, the solid line indicatingresults with no power control, the chain dashed line indicating resultswith power control having a single step size, and the dashed lineindicating results with power control having three step sizes.

In the drawings the same reference numerals have been used to indicatecorresponding features.

Referring to FIG. 1, a radio communication system which can operate in afrequency division duplex mode comprises a primary station (BS) 100 anda plurality of secondary stations (MS) 110. The BS 100 comprises amicrocontroller (μC) 102, transceiver means (Tx/Rx) 104 connected toantenna means 106, power control means (PC) 107 for altering thetransmitted power level, and connection means 108 for connection to thePSTN or other suitable network. Each MS 110 comprises a microcontroller(μC) 112, transceiver means (Tx/Rx) 114 connected to antenna means 116,and power control means (PC) 118 for altering the transmitted powerlevel. Communication from BS 100 to MS 110 takes place on a downlinkfrequency channel 122, while communication from MS 110 to BS 100 takesplace on an uplink frequency channel 124.

One embodiment of a radio communication system uses a scheme illustratedin simplified form in FIG. 2 for establishing a communication linkbetween MS 110 and BS 100. The link is initiated by the MS 110transmitting a request 202 (REQ) for resources on the uplink channel124. If it receives the request and has available resources, the BS 100transmits an acknowledgement 204 (ACK) on the downlink channel 122providing the necessary information for communication to be established.After the acknowledgement 204 has been sent, two control channels (CON)are established, an uplink control channel 206 and a downlink controlchannel 208, and an uplink data channel 210 is established fortransmission of data from the MS 110 to the BS 100. In some UMTSembodiments there may be additional signalling between theacknowledgement 204 and the establishment of the control and datachannels.

In this scheme separate power control loops operate in both uplink 124and downlink 122 channels, each comprising an inner and an outer loop.The inner loop adjusts the received power to match a target power, whilethe outer loop adjusts the target power to the minimum level that willmaintain the required quality of service (i.e. bit error rate). However,this scheme has the problem that when transmissions start on the controlchannels 206, 208 and data channel 210 the initial power levels andquality target are derived from open loop measurements, which may not besufficiently accurate as the channels on which the measurements weremade are likely to have different characteristics from the newlyinitiated channels. The result of this is that data transmissions at thestart of the data channel 210 are likely to be received in a corruptedstate if they are transmitted at too low a power level, or to generateextra interference if they are transmitted at too high a power level.

One known partial solution to this problem is for the BS 100 to measurethe received power level of the request 202 and to instruct the MS 110,within the acknowledgement 204, an appropriate power level for theuplink data transmission 210. This improves matters, but there may stillbe errors introduced by the temporal separation between the request 202and the start of the uplink data transmission 210.

FIG. 3 illustrates a solution to the problem in which the start of theuplink data transmission 210 is delayed by a time 302 sufficient for thepower control to have converged sufficiently to enable satisfactoryreception of data transmissions by the BS 100. A delay of one or twoframes (10 or 20 ms) is likely to be sufficient, although longer delays302 may be permitted if necessary. The additional overhead in thetransmission of extra control information on the control channels 206,208 is balanced by a reduced Eb/No (energy per bit/noise density) forthe user data received by the BS 100 over the data channel 210. Thedelay 302 could be predetermined or it could be determined dynamically,either by the MS 110 (which could detect convergence by monitoringdownlink power control information) or the BS 100.

FIG. 4 is a flow chart showing another solution to the problem in whichthe power control step size is variable. Since the power control erroris likely to be greatest at the start of a transmission or after an idleperiod, the optimum power control step size will be larger than thatused for normal operation

The method starts 402 with the beginning of the transmissions of thecontrol channels 206, 208 and the data channel 210 (or the beginning oftheir retransmission after an interruption). The difference between thereceived power and target power is then determined at 404. Next thepower control step size is tested at 406 to determine whether it isgreater than the minimum. If it is the power control step size isadjusted at 408 before adjustment of the power at 410. The change instep size could be deterministic, or based on previous power controladjustments or on some quality measurement. The power control loop thenrepeats, starting at 404.

In one embodiment it is preferred to set the power control step sizeinitially to a large value, then reduce it progressively until itreaches the value set for normal operation (which may be cell orapplication specific). Preferably the ratio between successive stepsizes is no more than two, to allow for the possibility of correctingerrors in transmission or due to other factors. The power control stepsize could be changed in both uplink 124 and downlink 122 channels.

As an example, consider an initial sequence of power control step sizes(in dB) of: 3.0, 2.0, 1.5, 1.0, 0.75, 0.75, 0.5, 0.5, 0.25, where 0.25dB is the minimum step size. Using this sequence with power controlsignals every 1 ms, an initial error of up to 10 dB could be correctedwithin half a frame (5 ms), compared with 2.5 frames using the minimumpower control step size of 0.25 dB that is normally used. Although asdescribed here the step sizes are symmetric (i.e. the same step sizesare applicable to increases or decreases in power), it is known (forexample from U.S. Pat. No. 5,056,109) that this is not alwaysappropriate. In a similar example, which would be simpler to implement,the initial step size (e.g. 2 dB) is used for a predetermined number ofpower control commands, after which the step size is reduced (e.g. to 1dB).

The selection of initial step size and the rate of change could bepredetermined, or determined dynamically. For example, if the powerlevel adjustment signalled in the acknowledgement 204 is large then theinitial step size could be increased. As another example, if the MS 110is able to determine by other means that it has a moderately high speedrelative to the BS 100 a larger step size may be appropriate.

A fixed power control adjustment could be applied at the start of thetransmission. This could be done even before receipt of any valid powercontrol command, but the size and direction might be predetermined ordetermined dynamically, for example, using information such as the rateof change of the channel attenuation derived from receiver measurements.Under some channel conditions this gives an improvement in performance.Increasing the power in this way is particularly suited to the case ofre-starting a transmission after an interruption, where the state of thepower control loop (e.g. current power level) may be retained frombefore the interruption. An interruption is a pause or gap intransmission during which time one or more of the control and datachannels are either not transmitted or not received (or both), but thelogical connection between the BS 100 and MS 110 is maintained. It couldbe either unintentional, caused by a temporary loss of signal, ordeliberate, typically because the MS 110 or BS 100 has no data totransmit or wishes to perform some other function such as scanningalternative channels.

In rapidly changing fading channels the channel attenuation following apause in transmission is likely to be uncorrelated with that immediatelybefore the pause. In such a case it may be argued that the optimum valueof the initial transmission power after the gap will be equal to itsaverage value (ignoring other slow fading effects like shadowing). Thiswill then minimise the difference between the initial value and theoptimum instantaneous value due to channel fluctuations. In practice, inone arrangement the transmission power after the gap is determined froma weighted average of the power over some extended period before thegap. A suitable averaging period would depend on particular conditionsbut could be of the order of 20 slots (i.e. 20 power control cycles). Anadditional offset or fixed power adjustment is optionally applied tothis initial power level. Optimum values of such offsets for particularcircumstances could be determined empirically.

In an alternative arrangement the initial power is determined from aweighted sum of power control commands, rather than measurement of thetransmitted power. In this arrangement the change in power (in dB) whichwould need to be applied after a transmission gap could, for example, becomputed recursively in the following way:ΔP(t)=P _(off) +K ₁×(ΔP(t−1)−P _(off))−K ₂ ×PC(t)×PS(t)where:

-   ΔP(t) is the change in power which would be applied after a gap,    computed recursively at time t, during active transmission;-   ΔP(0) could be initialised to zero;-   P_(off) is an additional power offset (which may be zero);-   K₁ and K₂ are empirically determined constants, which could be    equal, preferably such that 0≦K≦1. The values of these constants can    be chosen to reflect the effective averaging period used in    calculating the power change;-   PC(t) is power control command applied at time t; and-   PS(t) is the power control step size used at time t.

ΔP(t) is effectively the difference between the current power and aweighted average power, and should be quantised to an available powercontrol step size before it is used.

One example of an embodiment in which the selection of step size isdetermined dynamically uses the sign of the received power control bitsto determine the step size. When the MS 110 starts to receive powercontrol commands it uses the largest available step size, and continuesto use this step size until a power control command of opposite sign isreceived when the step size is reduced. This next step size is useduntil the sign of the power control commands is reversed, when the stepsize is again reduced. This process continues until the minimum stepsize is reached.

FIG. 5 is a graph showing the effect of this method in a system havingtwo step sizes available. The graph shows how the received signal power(P) in dB, relative to a target power of 0 dB, varies with time (T). Thesolid line shows the received signal power without use of power control.The variation in received power could for example be due to the motionof the MS 110. The chain-dashed line shows the received power with useof power control having a single step size of 1 dB. The dashed lineshows the received power with the use of power control in accordancewith the above method.

In this method, when use of power control begins, at about 4 ms, alarger step size of 2 dB is used. Initially the received power is lessthan the target power, so all the power control commands request anincrease in power and the 2 dB step size continues to be used.Eventually, at about 6 ms, the received power exceeds the target power.Once this happens the sign of the power control command reverses, torequest a decrease in power, which also has the effect of reducing thestep size to the standard step size of 1 dB. This step size thencontinues to be used in response to subsequent power control commands.

It is apparent from FIG. 5 that use of the described method enables thereceived power to reach its target more rapidly than is possible with asingle step size. Once the target has been reached, the reduction instep size to the standard step size enables accurate power control to bemaintained. Such a method enables cases where the initial error is largeor the channel is rapidly changing to be handled effectively, as well ascases where convergence is achieved quickly.

The method can also be used with a greater number of available stepsizes. FIG. 6 shows the same example as FIG. 5 with the exception thatthe dashed line shows the received power with the use of power controlhaving three step sizes, 4 dB, 2 dB and 1 dB, available. Initially a 4dB step size is used, with the result that the power reaches the targetmuch more rapidly than in the previous example. When the sign of thepower control command reverses, to request a reduction in power, thestep size is reduced to 2 dB. When the power control command reversesagain, to request an increase in power, the step size reduces to thestandard step size of 1 dB, where it remains.

A variation of the above method is to continue using the larger stepsize for one slot after the change in sign of the power control command,which could help to correct any overshoot. However, this is unlikely tohave a major impact on the average performance of the method.

Combinations of the techniques described above can readily be used toprovide improved results.

Although the description above has examined data transmission on theuplink channel 124, the techniques are equally applicable to datatransmission on the downlink channel 122 or to bidirectionaltransmissions.

Embodiments of the present invention have been described using spreadspectrum Code Division Multiple Access (CDMA) techniques, as used forexample in UMTS embodiments. However, it should be understood that theinvention is not limited to use in CDMA systems. Similarly, althoughembodiments of the present invention have been described assumingfrequency division duplex, the invention is not limited to use in suchsystems. It may also be applied to other duplex methods, for exampletime division duplex (although the power control rate in such a systemwould normally be limited to once per transmission burst).

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in radio communication systemsand component parts thereof, and which may be used instead of or inaddition to features already described herein.

In the present specification and claims the word “a” or “an” precedingan element does not exclude the presence of a plurality of suchelements. Further, the word “comprising” does not exclude the presenceof other elements or steps than those listed.

1. A radio communication system, comprising: a primary station; aplurality of secondary stations; a communication channel between theprimary station and a first secondary station, the communication channelincluding an uplink and a downlink control channel for transmission ofcontrol information, including power control commands, and a datachannel for the transmission of data; power control means for adjustingthe power of the control and data channels in response to the powercontrol commands; means for setting an initial transmission power afteran interruption in transmission to that before the interruption adjustedby an offset; and means for determining the offset from a differencebetween a last transmission power and a weighted average of thetransmission power over a predetermined period before the interruptionin transmission.
 2. The radio communication system as claimed in claim1, further comprising: means for determining the offset from a weightedsum of the power control commands applied before the interruption intransmission.
 3. The radio communication system as claimed in claim 1,further comprising: means for quantizing the offset to an availablepower control step size.
 4. A radio communication system, comprising: aprimary station; a plurality of secondary stations; a communicationchannel between the primary station and a first secondary station, thecommunication channel including an uplink and a downlink control channelfor transmission of control information, including power controlcommands, and a data channel for the transmission of data; power controlmeans for adjusting the power of the control and data channels inresponse to the power control commands; and means for setting an initialtransmission power after an interruption in transmission to that beforethe interruption adjusted by an offset; and means for determining theoffset from a weighted sum of power control commands in accordance withan equation ΔP(t)=K₁ΔP(t−1)−K₂PC(t)PS(t), where ΔP(t) is the offsetcomputed at a time t of a last power control command before theinterruption, ΔP(t−1) is a previously-determined offset, PC(t) is thepower control command applied at the time t, PS(t) is a size of thepower control step applied at the time t, K₁ and K₂ are constants andΔP(0) is set to zero at the start of the transmission; and means forquantizing the offset to an integer multiple of a minimum power controlstep size supported by the station transmitting the channel.
 5. Aprimary station for use in a radio communication system having acommunication channel between the primary station and a secondarystation, the channel including an uplink and a downlink control channelfor transmission of control information, including power controlcommands, and a data channel for the transmission of data, the primarystation comprising: power control means for adjusting the power of thecontrol and data channels in response to the power control commands; andmeans for setting an initial transmission power after an interruption intransmission to that before the interruption adjusted by an offset; andmeans for determining the offset from a difference between a lasttransmission power and a weighted average of the transmission power overa predetermined period before the interruption in transmission.
 6. Theprimary station as claimed in claim 5, further comprising: means fordetermining the offset from a weighted sum of the power control commandsapplied before the interruption in transmission.
 7. The primary stationas claimed in claim 5, further comprising: means for quantizing theoffset to an available power control step size.
 8. A primary station foruse in a radio communication system having a communication channelbetween the primary station and a secondary station, the channelincluding an uplink and a downlink control channel for transmission ofcontrol information, including power control commands, and a datachannel for the transmission of data, the primary station comprising:power control means for adjusting the power of the control and datachannels in response to the power control commands; and means forsetting an initial transmission power after an interruption intransmission to that before the interruption adjusted by an offset;means for determining the offset from a weighted sum of power controlcommands in accordance with an equation ΔP(t)=K₁ΔP(t−1)−K₂PC(t)PS(t),where ΔP(t) is the offset computed at a time t of a last power controlcommand before the interruption, ΔP(t−1) is a previously-determinedoffset, PC(t) is the power control command applied at the time t, PS(t)is a size of the power control step applied at the time t, K₁ and K₂ areconstants and ΔP(0) is set to zero at the start of the transmission; andmeans for quantizing the offset to an integer multiple of a minimumpower control step size supported by the primary station.
 9. A secondarystation for use in a radio communication system having a communicationchannel between the secondary station and a primary station, the channelincluding an uplink and a downlink control channel for transmission ofcontrol information, including power control commands, and a datachannel for the transmission of data, the secondary station comprising:power control means for adjusting the power of the uplink control anddata channels in response to the downlink power control commands; meansfor setting an initial transmission power after an interruption intransmission to that before the interruption adjusted by an offset; andmeans for determining the offset from a difference between a lasttransmission power and a weighted average of the transmission power overa predetermined period before the interruption in transmission.
 10. Thesecondary station as claimed in claim 9, wherein the offset ispredetermined.
 11. The secondary station as claimed in claim 9, furthercomprising: means for determining the offset from a weighted sum of thepower control commands applied before the interruption in transmission.12. The secondary station as claimed in claim 9, further comprising:means for quantizing the offset to an available power control step size.13. A secondary station for use in a radio communication system having acommunication channel between the secondary station and a primarystation, the channel including an uplink and a downlink control channelfor transmission of control information, including power controlcommands, and a data channel for the transmission of data, the secondarystation comprising: power control means for adjusting the power of theuplink control and data channels in response to the downlink powercontrol commands; means for setting an initial transmission power afteran interruption in transmission to that before the interruption adjustedby an offset; and means for determining the offset from a weighted sumof power control commands in accordance with an equationΔP(t)=K₁ΔP(t−1)−K₂PC(t)PS(t), where ΔP(t) is the offset computed at atime t of a last power control command before the interruption, ΔP(t−1)is a previously-determined offset, PC(t) is the power control commandapplied at the time t, PS(t) is the size of the power control stepapplied at the time t, K₁ and K₂ are constants and ΔP(0) is set to zeroat the start of a transmission or immediately after a gap, and in thatmeans are provided for quantizing the offset to an integer multiple of aminimum power control step size supported by the secondary station.